In automation technology, especially in process automation technology, field devices are often applied, which serve for determining, optimizing and/or influencing process variables. Serving for registering of process variables are sensors, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, respectively the fill level in a container, can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and deliver, or process, process relevant information. In connection with the invention, the terminology, field devices, thus includes also remote I/Os and radio adapters, and, in general, all devices, which are arranged at the field level. A large number of such field devices are manufactured and sold by the firm Endress+Hauser.
Decisive for an antenna apparatus are its dimensions relative to the wavelength. Other properties of antenna apparatuses are the degree of bundling, as well as the range, which separates near field from far field. A higher degree of bundling is equivalent to a smaller “aperture angle” of the transmitted electromagnetic rays. The degree of bundling determines how strongly an antenna can focus. When the antenna apparatus represents, for example, a larger TV antenna, the antenna apparatus has a smaller receiving angle range and can more exactly be directed at the transmitter. The higher the degree of bundling, the more parallel radiated wave fronts leave from an antenna. Moreover, there are other properties, such as, for example, broadbandedness, matching (less reflection), aperture, pressure resistance and (energy-)efficiency, which must be optimized simultaneously relative to one another.
The near field is, relative to the wavelength, the region in the immediate vicinity of an antenna apparatus and the far field is, relative to the wavelength, located a significant distance from the antenna apparatus. Far field means virtually no phase difference between electrical and magnetic fields and their oscillation directions are perpendicular to one another. This is especially advantageous for data connections over greater distances measured relative to the wavelength in the case of high data rates, such as, for example, mobile telephony, WLAN, directional radio links, Bluetooth, UMTS and LTE, since the radiated energy is radiated uniformly in the respectively desired one or more directions. Wave resistance depends on the properties of the atmosphere, respectively the surrounding material. The wave impedance for electrically non-conductive materials is the square root of the ratio of the complex permeability to the complex permittivity.
In the near field, there results from an evaluation of a Poynting vector in a case of transmission, an energy transmission back into the antenna apparatus, whereupon such is then radiated out again. A complex wave impedance results. The fraction of the energy coming directly back into the antenna apparatus can be selected by suitable dimensioning. In this way, transformers as well as NFC/RFID systems can be implemented within the near field range. In the case of RFID systems, the transmitted energy is sufficient to supply a small electronics unit, which contains, for example, a transmitter as well as other elements.